Curing Lights

A review of the characteristics of current devices.

By Allison C. Broyles, DMD, MS; and Jack Ferracane, PhD

One of the earliest types of curing lights introduced, the quartz-tungsten-halogen (QTH) light, is still used in many practices. These units consist of a tungsten filament inside a quartz bulb filled with halogen gas. When an electric current flows through the filament, it heats the tungsten, causing it to emit ultraviolet (UV) and white light. A filter is used to block all light except that between 400 nm and 550 nm. This violet-blue light is most efficiently absorbed by the photoinitiator, camphoroquinone (CQ). Because these units produce a lot of heat, a cooling fan is necessary to extend the lifetime of the bulb. The filter and fan can make these units heavier and more cumbersome than other widely available light-curing units. In addition, the bulb output decreases over time, making frequent replacement necessary.

The most popular type of curing device that has essentially been replacing the QTH lights is the light-emitting diode (LED). LEDs are made of two semiconductor crystals, one type “n” and the other type “p.” Each of these has a different electron density which means that an electric current will only flow in one direction between the diodes. When an electric current is passed through the crystals, energy is produced at the “np” junction and the energy is released in the form of light. The wavelength of light emitted is determined by the type of crystals used.3 LED lights are extremely efficient because, unlike QTH lights, all emitted light is within the blue region of the visible spectrum, and no filters are required. The diode is placed on a metallic heat sink to reduce its thermal rise during use, so in most cases a fan is not needed for cooling. While these devices generate less heat than QTH lights, devices with high light output can create considerable heat at the tip of the light guide. LED units are more stable than QTH bulbs and have very little decrease in bulb intensity over time. They are cost-effective, lightweight, and battery-powered for portability.6

The argon-ion laser made a brief appearance in dentistry but due to the cumbersome nature of the unit, the expense, the heat production, and the inability of auxiliary staff to use the unit in the United States, the units were not around for long. Plasma-arc curing lights (PAC lights) consist of two tungsten electrodes in a high-pressure gas-filled chamber. The electrodes are separated by a small gap and when light is reflected onto this area from a parabolic reflective surface, it creates a high electrical potential between the electrodes, which creates a spark. This spark ionizes the gas, providing a conductive path (a plasma) between the electrodes.6 Due to the rapid development of high-intensity LED units, the popularity of these PAC lights has been greatly reduced.

It is important to ensure that resin composite is being subjected to the appropriate total energy, or radiant exposure (J/cm2). Radiant exposure is the product of exposure time and irradiance (mW/cm2).5 Studies vary, but sufficient radiant exposure is likely in the range of about 15 J/cm2 to 20 J/cm2. The irradiance values of commercially available curing lights vary greatly, from 300 mW/cm2 to 4,000 mW/cm2.5 Complicating matters further, the light output, or irradiance, of curing lights may decrease over the life of the light-curing unit as well as with an increase in distance from the light-curing tip to the resin composite. Radiometers are available to monitor the irradiance from a light-curing unit. Unfortunately, some of these devices can be inaccurate and only measure the irradiance at the light-curing tip, rather than the total energy actually received by the resin composite (does not take into account the distance from the tip to the surface of the composite).5 However, it is still recommended to use these devices in a practice to monitor changes in a given curing unit over time.

Manufacturers use a variety of photoinitiators in resin composites and often there is more than one type of photoinitiator in a single product. Each photoinitiator may require a different light frequency to initiate polymerization; however, rarely does the manufacturer label the required frequencies. While this is not necessarily a problem when using a broad-spectrum light, it can complicate matters when using a LED light with a narrow output range. To address this, manufacturers have developed “polywave” lights, or LED lights that emit multiple frequencies. It is important to remember that the different colors of light emitted by a LED do not mix well within the beam and do not all penetrate the composite to the same degree. When using this type of light-curing unit, the clinician should slowly move the tip of the LED across the surface of the composite to ensure that all areas of the composite are being exposed to all frequencies of light.1

Teeth are subjected to increased temperatures during curing, not only while being exposed to the light-curing unit, but also by the heat generated in the polymerization reaction itself. It is widely understood that a rise in the temperature of a tooth can cause pulpal damage. The threshold at which such pulpal damage occurs is not known, but it is widely accepted that pulpal insult is cumulative over the life of a tooth and should be minimized whenever possible. While first-generation LEDs generated low levels of heat, those currently available often have very high power output and may generate a lot of heat. It may be beneficial to apply a steady airstream to the tooth during light-curing and to allow the tooth the return to its baseline temperature between cycles.

The shade and opacity of the composite are also important considerations.5 Darker and more opaque shades typically require longer curing times for a given thickness of composite. Recommended curing thicknesses are typically 2 mm, and it is reasonable to extend curing times by 50% for very dark shades, or reduce thickness by about 50%.

Retinal burning and advancement of macular degeneration are potential risks when using light-curing units. Therefore, it is very important for clinicians to use protective reddish-orange eyewear or shields that act as “blue blockers” to help prevent potential problems.1

Features of Current Curing Lights

Some of the most popular devices on the market are listed in Table 1. The features available include the following:

• Different handle designs
• Option for different irradiance levels
• Battery capacity (ie, cure time, varies from 30 to 60 minutes)
• Cordless (battery-powered) with the option for use with direct A/C current
• Curing timers with several time options
• Variable-intensity cure during the normal duty cycle (ie, pulsating)
• Attachable eye shield
• Interchangeable light guides of various diameters
• LEDs in the curing tip, ie, no light guide
• Radiometer built into the charging base
• Multiple wavelength output (more than one LED type)
• Alternative to continuous cure modes, such as ramp or pulse
• Sleep mode to save battery life
• Durability/shock resistance of housing (metal vs. plastic)
• Housing material may be from recycled material

Current and Future Concerns

In recent years more attention has been given to the energy efficient use of power. While this has contributed to the rise in the use of LED light-curing units, QTH light-curing units will continue to decrease in popularity because of the enhanced features of the LED units as well as the potential restrictions on the use of incandescent light bulbs. Both national and international standardization as well as manufacturing light-curing units with more uniform output characteristics is called for.